The term biomass is understood as meaning cultivated plants, biomass residue streams, such as prunings, trimmings and waste from parks and public gardens, and waste such as wood from demolition work and the unseparated biodegradable fraction of domestic refuse and industrial waste.
Gasification of biomass has to be distinguished from pyrolysis of biomass. Pyrolysis differs from gasification in that in pyrolysis no oxygen whatsoever is supplied and the process takes place at a lower temperature (400-700° C.). In both processes, gas (synthesis gas, syngas or fuel gas) and char are formed. The gas contains components which are substantially liquid at room temperature, known as tars. In the case of pyrolysis, there is a significant percentage of tars (approximately 65% by weight based on the feed) for which particular processes have been developed in order to substantially separate this large quantity of liquid tar as product oil from the product gas. U.S. Pat. No. 4,206,186 relates to the pyrolysis of a waste stream. In this pyrolysis described, oxygen or oxygen-containing compounds is/are supplied and there is generally less than 10% by weight of tar present.
The pyrolysis gas is in this case treated with a purge oil and then cooled to above the dew point of water. Adsorption or absorption then takes place with the aid of dry mass. As a result, harmful inorganic substances are removed and then the water is condensed out. The water which is condensed out then has to be subjected to a thorough cleaning step in order for residual tar and other oil-like components to be removed. This requires filtration through activated carbon.